Technical Field
The present disclosure relates to the field of imaging. More particularly, the present disclosure relates imaging using a complex laser. The present disclosure also relates to methods for adjusting spatial coherence of a complex laser.
Background Art
Many imaging applications require increasingly bright illumination sources, motivating the replacement of conventional thermal light sources with bright light emitting diodes (LEDs), superluminescent diodes (SLDs) and lasers. Despite their brightness, lasers and SLDs are poorly suited for full-field imaging applications because their higher spatial coherence leads to coherent artifacts such as speckle that corrupt image formation. See, e.g., Oliver, B. M. Sparkling spots and random diffraction, Proc IEEE 51, 220-221 (1963) and Goodman, J. W. Optical methods for suppressing speckle in Speckle phenomena in optics, 141-186 (Roberts & Company, 2007).
Lasers are indispensable light sources in modern imaging systems. Intense laser sources enable imaging through scattering or absorptive media and enable measuring dynamic behavior on short time scales. One of the signature properties of conventional lasers is high spatial coherence, a property resulting from resonant cavities with a limited number of spatial modes that produce well-defined wavefronts. A high-degree of spatial coherence has well-known advantages and disadvantages. On one hand, high spatial coherence allows for the highly directional emission of conventional lasers. On the other hand, spatial coherence leads to coherent imaging artifacts. Coherent artifacts originate from interference that occurs during image formation. The resulting intensity modulations appear as additional features that are not present in the object, thereby corrupting the image. Coherent artifacts can be introduced, for example, by aberrations in an imaging system or simply by diffraction when imaging objects with sharp edges. However, the most common manifestation of coherent artifacts is speckle, which occurs when a rough object or scattering environment introduces random phase delays among mutually coherent photons which interfere at the detector. See, e.g., Rigden, J. D. & Gordon, E. I., The granularity of scattered optical maser light, Proceedings of the Institute of Radio Engineers 50, 2367-2368 (1962). Speckle is a long-standing issue because it impairs image interpretation by a human observer. See, e.g., Geri, A. G. & Williams, L. A. Perceptual assessment of laser-speckle contrast, Journal of the Society for Information Display 20, 22-27 (2012); Gaska, J. P., Tai, C. & Geri, G. A. Laser-speckle properties and their effect on target detection Journal of the Society for Information Display 15, 1023-1028 (2007); and Artigas, J. M., Felipe, A. & Buades, M. J. Contrast sensitivity of the visual system in speckle imagery, J. Opt. Soc. Am. A 11, 2345-2349 (1994).
Over the years, various techniques have been developed to mitigate the effects of laser speckle by generating and averaging multiple uncorrelated speckle patterns (for instance, by scrambling the laser wavefront with a moving phase plate). See, e.g., McKechnie, T. S. Speckle reduction, in Topics in Applied Physics (ed. Dainty, J. C.) 9, 123-170 (Springer-Verlag, New York, N.Y., 1975). However, for M independent speckle patterns, speckle contrast (C) is reduced as M−1/2, fundamentally limiting the signal-to-noise ratio (1/C) of a measurement to the number of speckle patterns generated (rather than the detector integration time or photon statistics). See, e.g., Goodman, J. W. Optical methods for suppressing speckle in Speckle phenomena in optics, 141-186 (Roberts & Company, 2007). Hence, there is considerable interest in developing laser sources that fundamentally preclude the formation of coherent artifacts—that is, a laser with lower spatial coherence.
Imaging without coherent artifacts requires illumination of a sample with a large number of mutually incoherent photons. The number of photons per coherence volume (i.e. the photon degeneracy parameter) is therefore a relevant measure of source power since photons from distinct coherence volumes cannot interfere to generate coherent artifacts. From this perspective, the limitations of thermal sources and conventional lasers are clear. On one hand, thermal sources generate coherent artifact-free images (lower spatial coherence), but have very few photons per coherence volume (low photon degeneracy). On the other hand, conventional lasers have many photons per coherence volume (high photon degeneracy) but readily generate coherent artifacts (high spatial coherence). Thus, there is a need for sources with higher photon degeneracy and lower spatial coherence. This field relating to random lasers is relatively young and as such does not have many realized applications. To date, random lasers have not been adopted or applied in an imaging context.
Complex lasers support multiple spatial modes, either localized and/or extended. Examples of complex lasers include random lasers, partially ordered lasers, and chaotic cavity lasers. As described herein, complex laser may advantageously be designed with phase fronts that combine to produce emission with low or partial spatial coherence. This field relating to random lasers is relatively young and as such does not have many realized applications. To date, complex lasers have not been adopted or applied in an imaging context.
Random lasers are complex lasers in that they are made from disordered materials that trap light via multiple scattering. See, e.g., Cao, H. Lasing in Disordered Media, in Progress in Optics (ed. Wolf, E.) 45, 317-370 (North-Holland, Amesterdam, 2003) and D. S. Wierma, The physics and applications of random lasers, Nat. Phys. 4, 359-367 (2008). The spatial modes are inhomogeneous and highly irregular. With external pumping, a large number of modes can lase simultaneously with uncorrelated phases. Their distinctly structured wavefronts combine to produce emission with relatively low spatial coherence. Over the past two decades, random lasers have been the subject of intense theoretical and experimental studies. Id. Coherence is a fundamental characteristic of any laser, and, as such, the temporal coherence and second-order coherence of random lasers have been thoroughly investigated. See, e.g., V. M. Papadakis, A. Stassinopoulos, D. Anglos, S. H. Anastasiadis, E. P. Giannelis, and D. G. Papazoglou, J. Opt. Soc. Am. B 24, 31 (2007); M. A. Noginov, S. U. Egarievwe, N. Noginova, H. J. Caulfield, and J. C. Wang, Opt. Mat. 12, 127 (1999); H. Cao, Y. Ling, J. Y. Xu, C. Q. Cao, and P. Kumar, Phys. Rev. Lett. 86, 4524 (2001); G. Zacharakis, N. A. Papadogiannis, G. Filippidis, and T. G. Papazoglou, Opt. Lett. 25, 923 (2000); and M. Patra, Phys. Rev. A 65, 043809 (2002). Spatial coherence of random laser emission, however, is not well understood despite initial observations indicating that it is much lower than in a conventional laser.